Mr Petterson plans to introduce a bill before the Assembly this week so adults possessing up to 50 grams of cannabis or four cannabis plants would no longer be committing an offence.

A spokeswoman for ACT Labor confirmed the party was united in support of the bill, with 12 votes in favour.

“Whether the 13th vote will appear depends on the Greens and the Liberals,” she said.

But Greens parliamentarian Shane Rattenbury, whose vote would be required for the bill to pass, said while his party supported the bill in principle, he would wait until he had seen it and proposed some amendments before supporting it.

“We've long held the policy to keep people out of the criminal justice system for the possession of personal amounts of drugs,” he said.

“As a matter of principle we support this, and what we'll be looking at is whether we'll be making any amendments and further propositions in the bill.”

He said it would be worthwhile for the government to get a legal opinion on the bill, given there would be significant legal barriers to it becoming law, even if was passed by the ACT parliament.

“The bill will be tabled next week, so it can't be debated until next February at the soonest - there's plenty of time to have a look at it,” he said.

Possible legal barriers include the bill’s inconsistency with federal laws relating to cannabis, as well as the fact that the Commonwealth Poisons Standard lists cannabis as a prohibited substance.

ACT shadow attorney-general Jeremy Hanson said the Liberals would not support the bill.

“We support the current regime, where although there are penalties, small personal use has been decriminalised,” he said.

“We think that that strikes the right balance. We want to discourage marijuana use. It has a significant impact on psychosis, particularly for younger people and disadvantaged groups.”

He had also taken note of the Australian Medical Association’s submission to the draft bill. It did not support the legalisation of marijuana.

The association’s position is that cannabis should be seen “primarily as a health issue and not primarily as a matter of law enforcement”.

Mr Hanson said drug dealers would be the biggest beneficiaries of the bill should it become law.

“Aside from the quite significant mental health concerns, really the only winners then are organised crime and drug dealers,” he said.

“It's ill-considered. I think there are some other aspects to it. Points have been raised as to whether this would be in conflict with federal legislation, and I think it would be. We'll seek further advice on that, but when you have ACT legislation that is in conflict, then federal legislation takes precedence, so it would be struck out.

"Also, it is putting the ACT at odds with other jurisdictions. I don't think that's helpful.”

A spokesperson for federal Attorney-General Christian Porter said it was too early to determine the bill’s impact on Commonwealth law.

“It’s not really an issue he can speculate on until the ACT legislature has considered and made a decision,” he said.

Chief Minister Andrew Barr, meanwhile, reiterated his support on Friday.

"It's not a retail model - it simply would allow for personal use,” he said.

He pointed out that the ACT was not in a position to be able to put in a place a retailing system to allow buying and selling of cannabis like in the United States or Canada.

"If that was possible, then I would be supportive of that, but we can't, so we're not going down that path,” he said.

But he said it was open to the federal government change the laws.

“I think that will happen at some point in the future. But I don't think the rest of Australia is ready for that.”

He said making small amounts of cannabis legal for personal use would have an effect on the drug market in Canberra.

"It will certainly undercut the business model for those who have been in the business of supplying to people who will wish to use cannabis,” he said.

"This would be the single most effective measure in undermining the business model of organised crime for this particular drug. You take the money out of it, then suddenly the motivation for a whole range of criminal activity is removed.”

Mr Pettersson said the fact that he was introducing this bill now was in line with what he described as a global movement.

“If this law is passed, there will be no cannabis shops - it simply means people can grow and possess 50 grams of cannabis,” he said.

“I like to think of it as a reverse of the Scandinavian sex worker laws, where it is legal to sell sex services but it's not legal to buy them. This is the opposite. It would continue to be illegal to be a drug dealer, a drug supplier, but individuals who consume, we're not going to criminalise that behaviour.”